Explosive compositions comprising ammonium nitrate have been widely used throughout the world for many years. As ammonium nitrate is not readily detonatable in and of itself, it is typically mixed with carbonaceous fuels in order to obtain a mixture which is detonatable. Additional compounds such as sensitizers, densifiers, modifiers and surfactants may also be added to an ANFO explosive composition to improve various properties of the explosive composition including the sensitivity to detonation of the explosive, the energy of the explosion and the flowability of the explosive composition.
Typically, explosive compositions containing ammonium nitrate are manufactured at the location where they are to be utilized. For example, an ANFO explosive composition could be prepared at a mine and immediately loaded into a series of boreholes. The ANFO explosive composition would be loaded into the boreholes (typically from about 10 to 15 holes to more than about 100 holes) over a period of days. Typically, an ANFO explosive composition may be kept in a borehole anywhere from one hour up to fourteen days prior to being detonated. If the explosive is a prepackaged explosive composition, then due to shipping and handling time, the explosive composition must be stable for extended periods of time. A prepackaged explosive may also be stored for an extended period of time in a borehole prior to detonation. In some cases, the length of time between mixing the explosive composition and detonation of the explosive composition may be up to ninety days.
After being drilled, a borehole may remain dry for an extended period of time. However, in some cases, water will accumulate in boreholes, such as from the inflow of ground water. ANFO explosive compositions are adversely affected by water penetration and water absorption. Accordingly, if an ANFO explosive composition is loaded into a wet borehole or a borehole into which water subsequently seeps prior to detonation, then the ANFO explosive composition may deflagrate or, in fact, fail to detonate.
Typically, in order to increase resistance of the explosive composition to water, a gelling agent has been added to the explosive composition. The gelling agent may comprise guar gum, or guar gum and a mixture which includes, for example, sulphur and gilsonite (eg. ADTEC.TM.) which is added to the ammonium nitrate.
Additionally, U.S. Pat. No. 5,480,500 (Richard et al) discloses an improved water resistant ANFO explosive, having a wide particle size distribution as well as a gelling agent as above described. The wide particle size distribution, together with the gelling agent, increases the resistance of the explosive composition to water. However, this patent teaches that the addition of a gelling agent such as guar gum is necessary to achieve a satisfactory water resistant product. The theory is that in the wide particle size distribution, the small particles (eg. miniprills) fill some of the interstitial spaces in the larger ammonium nitrate particles. The gelling agent swells or hydrates upon contact with water to form a gel. The gel acts as a barrier which reduces the absorption of water by the ammonium nitrate particles, thus increasing the overall water resistance of the explosive composition.